1. Field of the Invention
This invention relates to a method of making a hologram lens, and more particularly to a method of making a hologram lens having a large effective diameter.
2. Description of the Prior Art
It is well known in the art that a hologram can be used as an optical element for converging a laser beam to a minute light spot as disclosed by J. M. Moran in Applied Optics, vol. 10, p. 412-415. The hologram lens is markedly thin and light in comparison with the conventional optical glass lens, and is very advantageous in that it is easily mass-produced.
In making a hologram lens which is used to form a real image, a diverging light beam and a converging light beam are used. In making a hologram lens used to form a virtual image, two diverging light beams are used. In making a hologram lens used for Fourier transformation, a collimated light beam and a converging light beam are used. Since the converging light beam and the collimated light beam are obtained by use of conventional glass lenses, the diameter of the light beam is limited by the diameter or aperture of the collimating lenses. Accordingly, a lens having a large effective diameter cannot be obtained in accordance with the prior art.
More quantitatively, the effective diameter D of a hologram lens is determined and accordingly limited by the following formula EQU D = (l.sub.2 /l.sub.1 + l.sub.2) D.sub.0 ( 1)
where l.sub.1 is the distance between the glass lens used to converge the laser beam and the hologram, l.sub.2 is the distance between the hologram and the converging point, and D.sub.0 is the diameter of the glass lens.
The hologram lens which is made in accordance with the above-mentioned method has a minimum length of resolution .delta. defined as follows: EQU .delta. = 1.22 .lambda.l.sub.2 /D
where .lambda. is the wavelength of the laser beam. Therefore, it is desired that the effective diameter D of the hologram lens be as large as possible in order to obtain as high resolution as possible.